feat: channel plugin architecture with decoupled configs

- Add plugin discovery via Python entry_points (group: nanobot.channels)
- Move 11 channel Config classes from schema.py into their own channel modules
- ChannelsConfig now only keeps send_progress + send_tool_hints (extra=allow)
- Each built-in channel parses dict->Pydantic in __init__, zero internal changes
- All channels implement default_config() for onboard auto-population
- nanobot onboard injects defaults for all discovered channels (built-in + plugins)
- Add nanobot plugins list CLI command
- Add Channel Plugin Guide (docs/CHANNEL_PLUGIN_GUIDE.md)
- Fully backward compatible: existing config.json and sessions work as-is
- 340 tests pass, zero regressions

tests/test_slack_channel.py

@@ -5,7 +5,7 @@ import pytest
 from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.channels.slack import SlackChannel
-from nanobot.config.schema import SlackConfig
+from nanobot.channels.slack import SlackConfig
 
 
 class _FakeAsyncWebClient: